Re: [netcdfgroup] NetCDF with gather compression, curvilinear cells, and projected coordinates - all at once! Possible?

  • To: Tim Cera <timcera@xxxxxxxxxxxxx>
  • Subject: Re: [netcdfgroup] NetCDF with gather compression, curvilinear cells, and projected coordinates - all at once! Possible?
  • From: John Caron <caron@xxxxxxxxxxxxxxxx>
  • Date: Wed, 15 Apr 2009 21:34:09 -0600
Hi TIm:

The IDV/CDM software doesnt handle CF compressed indices. As you have 
discovered, no one else does either, and there has been a suggestion to 
deprecate this feature, since netcdf4 compression might work just as well, in a 
way that would be transparent to applications. Would you like to try that?

John




Tim Cera wrote:
> I finally sat down to try and output to NetCDF from the Environmental
> Fluid Dynamics Code (EFDC) that we use for estuarine modeling.
> 
> EFDC already handles all computation in a gather compression mode so
> that would seem to be a good fit to have, plus only about 10% of the
> cells are wet cells which would make a big impact on file size if I kept
> the blank cells about.
> 
> EFDC uses a ordered, boundary-fitted, curvilinear, 4-sided cell grid
> with sigma-stretched layers in the vertical.  That is a mouthful.
> 
> We use UTM for the coordinates so the NetCDF needs to have projection
> information.
> 
> It seems like I have tried everything and I can't get any NetCDF tool to
> bring in this dataset.  I have tried IDV, Panoply, ncview (doesn't
> really support projection data), ncBrowse, and maybe some others.
> 
> IDV does the best, at least putting the projected coordinates near the
> right spot in the 'Region' map, but IDV thinks that the WSElevation data
> is (3013, 3013) instead of (259, 121).
> 
> I have 4 warnings when using the CF Compliance checker:
> 
> 
> WARNING (7.1): Data for variable y lies outside cell boundaries
> WARNING (7.1): Data for variable x lies outside cell boundaries
> ...
> ------------------
> Checking variable: rgrid
> ------------------
> WARNING (3): No standard_name or long_name attribute specified
> WARNING (3.1): units attribute should be present
> 
> 
> The first two I can fix because it must be related to the algorithm to
> calculate the cell corners.  I have checked the last few cells and they
> are correct.  The 'rgrid' variable is my gather compression variable and
> it shouldn't have units, nor could I find a reasonable standard name.
> 
> Here is the CDL:
> 
> 
>   netcdf efdcout {
> dimensions:
>         IC = 259 ;
>         JC = 121 ;
>         rgrid = 3013 ;
>         nv = 4 ;
>         time = UNLIMITED ; // (2 currently)
> variables:
>         int transverse_mercator ;
>                 transverse_mercator:grid_mapping_name =
> "transverse_mercator" ;
>                 transverse_mercator:latitude_of_projection_origin = 0. ;
>                 transverse_mercator:longitude_of_central_meridian = -81. ;
>                 transverse_mercator:false_easting = 500000. ;
>                 transverse_mercator:false_northing = 0. ;
>                 transverse_mercator:scale_factor_at_central_meridian =
> 0.9996 ;
>         int rgrid(rgrid) ;
>                 rgrid:compress = "IC JC" ;
>         float WSElevation(time, rgrid) ;
>                 WSElevation:standard_name =
> "height_above_reference_ellipsoid" ;
>                 WSElevation:units = "m" ;
>                 WSElevation:coordinates = "x y" ;
>                 WSElevation:grid_mapping = "transverse_mercator" ;
>         float x(rgrid) ;
>                 x:standard_name = "projection_x_coordinate" ;
>                 x:units = "m" ;
>                 x:bounds = "x_bnds" ;
>         float y(rgrid) ;
>                 y:standard_name = "projection_y_coordinate" ;
>                 y:units = "m" ;
>                 y:bounds = "y_bnds" ;
>         float x_bnds(rgrid, nv) ;
>         float y_bnds(rgrid, nv) ;
>         double time(time) ;
>                 time:standard_name = "time" ;
>                 time:units = "days since 1995-1-1 0:0:0" ;
> 
> // global attributes:
>                 :Conventions = "CF-1.0" ;
> }
> 
> Any ideas on what I am doing wrong?
> 
> Kindest regards,
> Tim Cera
> 
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